PHARRELL WILLIAMS: (Singing) Because I'm happy, clap along if you feel like a room without a roof. Because I'm happy, clap along...

SIEGEL: This is the song "Happy" from Pharrell Williams. He sings. He writes. He produces. Williams is also the creative force behind an ambitious new music video, though calling it just a video hardly does it justice.

MELISSA BLOCK, HOST:

That's because this video for "Happy" is 24 hours long. It hit the Web two weeks ago at the site 24hoursofhappy.com. It looks like an ordinary music video. There's lip-syncing to the song and someone dancing through the streets of Los Angeles. But here's where the ambition comes in. The song ends...

SIEGEL: But the video doesn't. The song starts over. The video picks up in the same location with a new happy dancer. The hand-off happens over and over again through the course of an entire day. The song plays 15 times per hour. Multiply that by 24 and you get a lot of happiness.

BLOCK: And a lot of work to put together.

MIMI VALDES: It was crazy and awesome at the same time.

(LAUGHTER)

BLOCK: That's Mimi Valdes. She's the creative director for Pharrell Williams' entertainment company. She was on-set every day for the 11-day shoot. She says the video involved over 400 extras. Many of the dancers they cast ahead of time, some they didn't.

VALDES: When we were filming in the streets of LA, sometimes we'd see really amazing, cool people that just seemed like, you know, they would fit in and - or people just watching. We were like, hey, do you want to join? And, of course, people, sort of jumped at the chance.

SIEGEL: While it's a catchy song, we had to ask Valdes: During the shoot, did you ever get tired of hearing it?

VALDES: I promise you, no. It was the weirdest thing. And I swore I was going to get sick of it, and I really didn't because every time it played, it felt like a new experience.

SIEGEL: And so it is with the finished product. You can browse and - if you don't like one dance number - you can skip to the next, 24 hours of happy to choose from.

(SOUNDBITE OF SONG, "HAPPY")

WILLIAMS: (Singing) If you feel like a room without a roof. Because I'm happy, clap along if you feel...